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From: dana@thinman.cray.com (Dana Dawson)
Newsgroups: comp.dcom.sys.cisco
Subject: Re:  minimum maximum transfer unit.
Message-ID: <34846@boulder.Colorado.EDU>
Date: 8 May 91 00:30:15 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 20

> Sort of except that MTU is the Maximum Transfer Unit, not the bandwidth.
> The MTU in your example would probably be limited by the ethernet to a
> 1500 byte packet.  Even though the FDDI or the T1 might be able to
> handle larger packets that is the largest that one can send without
> having to worry about fragmentation.
> 
> There is also a "minimum bandwidth" which would be the 1.544
> Megabits/s that you mention.  The T1 line being the slowest and thus
> establishing the minimum.

Some (many?) IP implementations compare the source and destination IP
addresses and use the interface MTU for "local" destinations (source and
destination addresses have matching network parts), and 576 for non-local
destinations because that is the smallest "required" MTU (see RFC 1122,
"Requirements for Internet Hosts -- Communication Layers", page 56).
This way, in theory, you "know" that no gateway along the way will have
to fragment your datagrams.  Do ciscos do a similar thing?

Dana Dawson
Cray Research, Inc.